Molding die apparatus, method for molding disc substrate, and disc-shaped recording medium

ABSTRACT

Preventing a molded disc substrate from generating a separation pattern. To this end, a molding die  1  includes a pair of stationary die  6  and movable die  7 , and an outer circumference ring  16  for molding an outer circumference end of the disc substrate  2 . The outer circumference ring  16  is so mounted as to be movable along forward and backward directions in parallel with a moving direction of the movable die  7  toward the stationary die  6 , and has a retaining surface  39  for retaining the outer circumference of the molded disc substrate  2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a molding die apparatus for molding adisc substrate of a disc-shaped recording medium such as an optical discand magneto-optical disc by injection molding, a method for molding adisc substrate, and a disc-shaped recording medium.

2. Description of the Related Art

Generally, a disc substrate or a basal plate of a disc-shaped recordingmedium such as an optical disc and magneto-optical disc is molded byinjection molding by the use of a molding die.

Such a molding die is required to suppress generation of burrs aroundthe outer circumference of a molded disc substrate, and to give off gasuniformly when a disc substrate is being molded.

The molding die includes a pair of stationary die and movable die whichhave a cavity for molding a disc substrate, an outer circumference ringfor molding an outer circumference end of a disc substrate, a stamperfor transcribing predetermined convexo-concave patterns onto a discsubstrate, a driving means for moving the movable die toward and awayfrom the stationary die.

There is formed a stationary side mirror-finished surface at thestationary die on which the stamper is to be mounted. On the other hand,there is formed a movable side mirror-finished surface (referred to as amolding surface, hereinafter) at the movable die, which molds a mainsurface of a disc substrate. And, there is mounted the outercircumference ring on the outer circumference of the molding surface ofthe movable die such that the outer circumference ring can move towardthe cavity substantially in parallel with the moving direction of themovable die.

The stamper is of a disc-shaped configuration having a center hole, andis abutted to the stationary side mirror-finished surface of thestationary die. The stamper has its inner circumference fixed by aninner stamper holder and has its outer circumference fixed by an outerstamper holder, respectively.

The outer circumference ring is substantially of a ring-shapedconfiguration, and there is formed a molding surface for molding anouter circumference end of a disc substrate on the inner circumferencethereof. The outer circumference ring is moved by an elastic body suchas a spring, or a moving means using air pressure, oil pressure, etc.

As shown in FIG. 1, when a movable die 107 is moved toward a stationarydie 106 and a molding die having the stationary die 106 and movable die107 is clamped, there is formed a cavity 108 enclosed by a moldingsurface 126 of the movable die 107, a main surface of a stamper 113abutted to the stationary die 106, and a molding surface 135 of an outercircumference ring 116, which space is to be filled with resin material.

At the time of clamping (closing) the molding die, the outercircumference ring 116 is moved toward the stationary die 106 with apredetermined pressure. The molding die is configured such that, whenthe outer circumference of the outer circumference ring 116 is abuttedto an outer stamper holder 115, there is secured a clearance gap 120 forevacuating gas in the cavity 108 between the outer circumference ring116 and the stamper 113. The clearance gap 120 is designed to have apredetermined size, which size can evacuate only gas in the cavity 108.

When injecting molten resin material into the cavity 108, air in thecavity 108 and gas generated from the molten resin material is evacuatedfrom the clearance gap 120. On filling the cavity 108 with molten resinmaterial, the injection of molten resin material is terminated. At thetime of injecting molten resin material, only air and gas is evacuatedfrom the clearance gap 120, and molten resin material is prevented fromflowing into the clearance gap 120 so as to suppress generation of burrsat a molded disc substrate.

In injecting molten resin material into the cavity 108, in case theinjection pressure is larger than the clamping force, the intervalbetween the stationary die 106 and the movable die 107 is caused to belarge. On the other hand, the pressure for causing the outercircumference ring 116 to protrude into the cavity 108 is being keptapplied. So, the clearance gap 120 will not be expanded, and generationof burrs can be suppressed.

After completing the injection of molten resin material and cooling downthe resin material, opening of the molding die is initiated. At thistime, the molded disc substrate is moved together with the moldingsurface 126 of the movable die 107.

Then, the molded disc substrate is separated from the molding surface126 of the movable die 107 by sleeve-shaped extruding pins (not shown)and ejected. Thus, the whole molding process composed of a series ofmolding steps is completed.

After opening the molding die, when the molded disc substrate isseparated from the molding surface 126 of the movable die 107, itdepends on the configuration of the molding die or molding conditionwhether the whole main surface of the disc substrate is attached to orreleased from the molding surface 126 of the movable die 107.

In case the whole main surface of the disc substrate is attached to themolding surface 126 of the movable die 107, separation resistance isprone to be large when the disc substrate is ejected. So, for example,an assistant means for supplying air is used to compulsorily separatethe disc substrate. However, it is difficult to spray air upon the wholemain surface of the disc substrate uniformly. That is, air isundesirably sprayed upon the center portion of the disc substratepartially where air can easily flow, which results in ill-balancedseparation of the disc substrate. So, it is required that, after themolding die is opened, the whole main surface of the disc substrate isreleased uniformly from the molding surface 126 of the movable die 107.

However, when the outer circumference ring 116 is moved such that itfurther protrudes into the cavity 108 from the molding surface 126 ofthe movable die 107, in case a pressure caused by the outercircumference ring 116 is not transmitted to the disc substrate, thewhole main surface of the disc substrate cannot be released from themolding surface 126 of the movable die 107.

In reality, in case a disc substrate (to be indicated with a referencenumeral of 102) is molded using a molding die having the outercircumference ring 116 shown in FIG. 1, a pressure caused by the outercircumference ring 116 cannot be transmitted to the disc substratesufficiently. So, the outer circumference of the disc substrate isunstably released, and the inner circumference of the disc substrateremains abutted to the molding surface 126 of the movable die 107. Thus,there arises a problem that a separation pattern 122 is generatedbetween the outer circumference and the inner circumference of the discsubstrate 102, as shown in FIG. 2. That is, only the outer circumferencering 116 protrudes toward the side of the disc substrate 102, and theinner and middle circumference of the disc substrate 102 remain abuttedto the molding surface 126 of the movable die 107, and thus theseparation pattern 122 is generated.

Thus molded disc substrate 102 with the separation pattern 122 not onlyspoils the beauty of a disc but also makes the optical and electricalcharacteristics thereof unstable. That is, for example, there arisewobbles in recording/reproducing signals.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theabove-mentioned drawbacks by providing a molding die apparatus which canrelease the whole main surface of a molded disc substrate uniformly froma molding surface at the time of opening the molding die apparatus so asto prevent the molded disc substrate from generating a separationpattern at the time of separating the molded disc substrate from themolding die apparatus, a method for molding a disc substrate using themolding die apparatus, and a disc-shaped recording medium having a discsubstrate molded by the molding die apparatus.

According to the present invention, there is provided a molding dieapparatus including:

a pair of stationary die and movable die for molding a disc substrate ofa disc-shaped recording medium; and

an outer circumference ring for molding an outer circumference end ofthe disc substrate;

the outer circumference ring being so mounted as to be movable alongforward and backward directions in parallel with a moving direction ofthe movable die toward the stationary die, and having a retainingsurface for retaining the outer circumference of the molded discsubstrate.

According to the molding die apparatus of the present invention, sincethe molded disc substrate has its outer circumference retained by theretaining surface of the outer circumference ring, when the outercircumference ring is moved along forward and backward directions, themolded disc substrate is moved together with the outer circumferencering.

Thus, according to the molding die apparatus of the present invention,the disc substrate can be prevented from generating a separation patternon the main surface thereof.

According to the present invention, there is provided a method formolding a disc substrate by injection molding by the use of a pair ofstationary die and movable die for molding a disc substrate of adisc-shaped recording medium, and an outer circumference ring formolding an outer circumference end of the disc substrate which is somounted as to be movable along forward and backward directions inparallel with a moving direction of the movable die toward thestationary die, the method for molding a disc substrate including thesteps of:

moving the outer circumference ring along backward direction away fromthe stationary die to cause the outer circumference ring to protrude bya protrusion length which is approximately equal to the thickness of thedisc substrate so as to perform injection molding at the time of dieclamping;

moving the outer circumference ring along forward direction toward thestationary die to cause the outer circumference ring to protrude so asto separate a main surface of the disc substrate by a predeterminedamount from a molding surface of the movable die for molding the mainsurface of the disc substrate by the use of a retaining surface forretaining the outer circumference of the molded disc substrate which isformed at the inner circumference of the outer circumference ring whendie opening is initiated; and

ejecting the disc substrate from the molding surface of the movable dieafter the die opening is finished.

According to the method for molding a disc substrate of the presentinvention, since the molded disc substrate has its outer circumferenceretained by the retaining surface of the outer circumference ring, themolded disc substrate is moved together with the outer circumferencering and the main surface of the disc substrate is separated from themolding surface of the movable die by moving the outer circumferencering to cause it to protrude toward the stationary die when die openingis initiated. Then, the molded disc substrate is ejected from themolding surface of the movable die after the die opening is finished.

Thus, according to the method for molding a disc substrate of thepresent invention, the disc substrate can be prevented from generating aseparation pattern on the main surface thereof.

According to the present invention, there is provided a disc-shapedrecording medium which has a retaining surface being of a continuous ordiscontinuous ring-shaped surface around the outer circumference thereofwhich is formed by an outer circumference ring of a molding dieapparatus for molding an outer circumference end of a disc substrate,and is retained by the outer circumference ring.

According to the disc-shaped recording medium of the present invention,since the molded disc substrate has its outer circumference retained bythe retaining surface of the outer circumference ring, the molded discsubstrate has its main surface satisfactorily separated from the movabledie.

Thus, according to the disc-shaped recording medium of the presentinvention, being prevented from generating a separation pattern on themain surface thereof, the optical and electrical characteristics of thedisc substrate can remain stable and desirably secured.

These objects and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the preferred embodiments of the present invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal sectional view for explaining an outercircumference ring of the conventional molding die.

FIG. 2 shows a plan view of a separation pattern generated on the mainsurface of a disc substrate molded by the conventional molding die.

FIG. 3 shows a longitudinal sectional view for explaining the wholeconfiguration of the molding die according to the present invention.

FIG. 4 shows a longitudinal sectional view for explaining the principalportion of the molding die according to the present invention.

FIG. 5 shows a longitudinal sectional view for explaining an outercircumference ring of the molding die according to the presentinvention.

FIG. 6 shows a longitudinal sectional view for explaining a state inwhich the molding die according to the present invention is clamped.

FIG. 7 shows a longitudinal sectional view for explaining a state inwhich the molding die according to the present invention is opened.

FIG. 8 shows a longitudinal sectional view for explaining another outercircumference ring of the molding die according to the presentinvention.

FIG. 9 shows a longitudinal sectional view for explaining yet anotherouter circumference ring of the molding die according to the presentinvention.

FIG. 10 shows a longitudinal sectional view for explaining a state inwhich the molding die according to the present invention using yetanother outer circumference ring is clamped.

FIG. 11 shows a longitudinal sectional view for explaining a state inwhich the molding die according to the present invention using yetanother outer circumference ring is opened.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The molding die apparatus according to the present invention for moldinga disc substrate of a magneto-optical disc by injection molding willfurther be described below with reference to the accompanying drawings.

The molding die apparatus according to the present invention will beemployed in molding a disc substrate having a diameter of 130 mm to beused as a basal plate of a magneto-optical disc conforming, for example,to the ISO/IEC1528 standard. The magneto-optical disc has a recordingcapacity of 5.2 GB (giga byte).

A molding die 1 according to the present invention includes a pair ofstationary die 6 and movable die 7 which configure a cavity 8 formolding a disc substrate 2, a stationary die holder 11 for holding thestationary die 6, a movable die holder 12 for holding the movable die 7,a stamper 13 for transcribing predetermined convexo-concave patternsonto the disc substrate 2, an inner stamper holder 14 for fixing theinner circumference of the stamper 13 to the stationary die 6, an outerstamper holder 15 for fixing the outer circumference of the stamper 13to the stationary die 6, and an outer circumference ring 16 for moldingan outer circumference end of the disc substrate 2, as shown in FIG. 3.

Furthermore, the molding die 1 includes a frame 17 for guiding themovable die 7 toward the stationary die 6, a spacer 18 for adjusting thesize of the cavity 8, a sleeve 19 for ejecting the disc substrate 2, apunch 20 for cutting off a gate around a center hole of the discsubstrate 2, and a pin 21 for ejecting the center hole portion cut offby the punch 20 and a spool runner, as shown in FIG. 3.

There is formed a stationary side mirror-finished surface 25 at thestationary die 6 on which the stamper 13 is to be mounted, as shown inFIG. 3. The movable die 7 is so mounted as to face the stationary die 6and be movable toward the stationary die 6 along forward and backwarddirections shown by arrows a₁ and a₂ in FIG. 3. On the other hand, thereis formed a movable side mirror-finished surface 26 (referred to as amolding surface 26, hereinafter) at the movable die, which molds a mainsurface of the disc substrate 2. And, there is formed a temperatureadjustment water channel 28 which causes chilled water for cooling downthe cavity 8 to flow at each of the stationary die 6 and movable die 7.

There is formed a spool bush 29 into which molten resin materialinjected by a screw of an injection molding apparatus, not shown, flows.As the resin material, polycarbonate, PMMA (polymethylmethacrylate), etcis used. The sleeve 19 is movably mounted to the center of the movabledie holder 12 and movable die 7.

The stamper 13 is of a disc-shaped configuration having a center hole,and has predetermined convexo-concave patterns such as pre-grooves andpre-formatted pits formed on a main surface thereof which face thecavity 8. The stamper 13 is abutted to the stationary sidemirror-finished surface 25 of the stationary die 6, and has its innercircumference pressed by the inner stamper holder 14 and its outercircumference pressed by the outer stamper holder 15, respectively,toward the stationary side mirror-finished surface 25 and fixed.

The inner stamper holder 14 is substantially of a cylindricalconfiguration, and has a pawl 33 for retaining the inner circumferenceof the stamper 13 at the outer circumference thereof. The outer stamperholder 15 is substantially of a cylindrical configuration, and has apawl 34 for retaining the outer circumference of the stamper 13 at theinner circumference thereof.

The outer circumference ring 16 is substantially of a ring-shapedconfiguration, and is located at outside the outer circumference of themolding surface 26 of the movable die 7, as shown in FIGS. 3 and 4. Theouter circumference ring 16 is so mounted to the movable die 7 as to bemovable toward the stationary die 6 along the forward and backwarddirections shown by arrows a₁ and a₂. The outer circumference ring 16has its one end movably supported by the movable die 7 via asubstantially ring-shaped spacer 32 for adjusting the movement of theouter circumference ring 16 along the forward direction shown by thearrow a₁.

There is formed a molding surface 35 for molding the outer circumferenceend of the disc substrate 2 on the inner circumference of a portion ofthe outer circumference ring 16 which is adjacent to the stamper 13, asshown in FIGS. 4 and 5. Thus, the molding die 1 has the cavity 8enclosed by a main surface of the stamper 13 abutted to the stationarydie 6, the molding surface 26 of the movable die 7, and molding surface35 of the outer circumference ring 16. The cavity 8 has its sizeappropriately adjusted when the thickness of the spacer 18 iscontrolled.

There is formed a retaining portion 36 for retaining the outercircumference of the disc substrate 2, which is located halfway at themolding surface 35 along the thickness direction of the molded discsubstrate 2.

The retaining portion 36 is of a step-shaped configuration along thethickness direction of the disc substrate 2. The retaining portion 36has a vertical surface 37 perpendicular to the main surface of the discsubstrate 2, an inclined surface 38 which is so formed as to be inclinedalong the thickness direction of the disc substrate 2, and a retainingsurface 39 formed in parallel with the main surface of the discsubstrate 2 between the vertical surface 37 and the inclined surface 38,as shown in FIG. 5. The vertical surface 37 is so formed as to beadjacent to the molding surface 26 of the movable die 7.

The inclined surface 38 is so formed as to face the stamper 13 and makethe diameter of the molded disc substrate 2 gradually increased whengetting closer the stamper 13. The inclined surface 38 of the outercircumference ring 16 can allow the disc substrate 2 to be separatedfrom the molding surface 26 of the movable die 7 with smaller separationresistance, which can prevent deformation of the disc substrate 2.

The retaining surface 39 is of a continuous ring-shaped surface aroundthe outer circumference of the disc substrate 2. The retaining surface39 may be of a discontinuous ring-shaped surface around the outercircumference of the disc substrate 2.

In case the disc substrate 2 having a diameter of 130 mm is to bemolded, the inclined surface 38 is formed such that the height h alongthe thickness direction of the disc substrate 2 from the retainingsurface 39 to an end of the portion of the outer circumference ring 16adjacent to the stamper 13 becomes approximately 1 mm and the angle ofinclination θ from the thickness direction of the disc substrate 2becomes approximately 5 degrees, as shown in FIG. 5. In this case, theretaining surface 39 is formed such that the width w along a directionperpendicular to the thickness direction of the disc substrate 2 fromthe beginning of the inclined surface 38 to the vertical surface 37becomes approximately 0.3 mm. And, the cavity 8 is adjusted by thespacer 18 such that the interval between the molding surface 26 of themovable die 7 and the main surface of the stamper 13 becomes a littleless than 1.2 mm.

At the time of opening the molding die 1, the outer circumference ring16 can be moved along the forward direction shown by the arrow a₁ by amoving means, not shown, using air pressure, etc. such that the moldingsurface 35 protrudes from the molding surface 26 of the movable die 7 bythe maximum protrusion length t₁ which is lager than the thickness ofthe disc substrate 2, as shown in FIG. 5. On the other hand, at the timeof clamping the molding die 1, since the outer circumference of theouter circumference ring 16 is abutted to the outer stamper holder 15,the outer circumference ring 16 is pressed and moved toward the movabledie 7 along the backward direction shown by the arrow a₂ such that themolding surface 35 protrudes from the molding surface 26 of the movabledie 7 by a predetermined protrusion length t₀ which is approximatelyequal to the thickness of the disc substrate 2, as shown in FIG. 6.

In case the disc substrate 2 having a thickness of 1.2 mm is to bemolded, the outer circumference ring 16 has its maximum protrusionlength t₁ set to be 1.4 mm to 1.7 mm. Thus, the outer-circumference ring16 protrudes by the predetermined protrusion length t₀ of 1.2 mm whichis approximately equal to the thickness of the disc substrate 2 when themolding die 1 is clamped, and is moved along the forward direction shownby the arrow a₁ by 0.2 mm to 0.5 mm at the time of opening the moldingdie 1.

The outer circumference ring 16 is configured such that, at the time ofclamping the molding die 1, there is secured a clearance gap 41 forevacuating gas in the cavity 8 between the end of the portion of theouter circumference ring 16 which faces the main surface of the stamper13 and the main surface of the stamper 13, as shown in FIG. 6.

The clearance gap 41, which is generally set to be 5 μm to 40 μm, is setto be approximately 20 μm by the outer circumference ring 16. So, moltenresin material is surely prevented from flowing into the clearance gap41, which suppresses generation of burrs around the outer circumferenceof the molded disc substrate 2.

As has been described above, the molding surface 35 of the outercircumference ring 16 is formed such that the inclined surface 38 andretaining surface 39 form an obtuse angle. On the other hand, themolding surface 35 of the outer circumference ring 16 may be formed suchthat the inclined surface 38 and retaining surface 39 form a curvedsurface without forming an angle, as shown in FIG. 8.

As has also been described above, the outer circumference ring 16 hasthe inclined surface 38 on its one end adjacent to the stamper 13. Onthe other hand, the outer circumference ring 16 may have a verticalsurface perpendicular to the main surface of the disc substrate 2 on itsone end adjacent to the stamper 13.

FIG. 9 shows another example of an outer circumference ring, which willbe indicated with a reference numeral of 61. As shown, there is alsoformed a retaining portion 62 for retaining the outer circumference ofthe disc substrate 2.

The retaining portion 62 is of a step-shaped configuration along thethickness direction of the disc substrate 2. The retaining portion 62has a first vertical surface 63 perpendicular to the main surface of thedisc substrate 2, a second vertical surface 64 perpendicular to the mainsurface of the disc substrate 2, and a retaining surface 65 formed inparallel with the main surface of the disc substrate 2 between the firstvertical surface 63 and the second vertical surface 64, as shown in FIG.9. The first vertical surface 63 is so formed as to be adjacent to themolding surface 26 of the movable die 7. The second vertical surface 64is so formed as to be adjacent to the main surface of the stamper 13,and is located at the outer circumference of the first vertical surface63.

Since the outer circumference ring 61 has the second vertical surface 64with no inclination, separation resistance caused at the time ofejecting the disc substrate 2 from the molding surface 26 of the movabledie 7 becomes undesirably large compared with the case in whichabove-described outer circumference ring 16 having the inclined surface38 is used. So, it is desired that the outer circumference ring has aninclined surface like the outer circumference ring 16 having theinclined surface 38.

The configuration of forming an inclined surface at an outercircumference ring is disclosed in, for example, Japanese Laying-OpenPatent H-3-283114. However, the outer circumference ring disclosed inthe Japanese Laying-Open Patent H-3-283114 has only the inclined surfaceat its retaining portion. So, since the outer circumference of the discsubstrate cannot be retained sufficiently, a pressure for ejecting thedisc substrate cannot be transmitted to the disc substrateappropriately.

Next, parts for forming a center hole of the disc substrate 2 will beexplained. The sleeve 19 is substantially of a cylindricalconfiguration, and has its one end caused to face the innercircumference of the disc substrate 2, as shown in FIG. 3. The sleeve 19is so mounted as to be movable along the forward and backward directionsshown by arrows a₁ and a₂. The punch 20 is substantially of acylindrical configuration, and has its one end caused to face the cavity8. The punch 20 is mounted inside the inner circumference of the sleeve19, and is so mounted as to be movable along the forward and backwarddirections shown by arrows a₁ and a₂. The pin 21 is substantially of arod configuration. The punch 20 is mounted inside the innercircumference of the punch 20, and is so mounted as to be movable alongthe forward and backward directions shown by arrows a₁ and a₂.

Next, the movement of the outer circumference ring 16 at the time ofclamping and opening the molding die 1, and the method for molding thedisc substrate 2 will be explained.

At the time of opening the molding die 1, the molding surface 35 of theouter circumference ring 16 is moved along the forward direction shownby the arrow a₁, such that the molding surface 35 protrudes from themolding surface 26 of the movable die 7 by the maximum protrusion lengtht₁ which is lager than the thickness of the disc substrate 2, as shownin FIG. 5.

Then, the molding die 1 is clamped when the movable die 7 is movedtoward the stationary die 6 along the forward direction shown by thearrow a₁. At the time of clamping the molding die 1, since the outercircumference of the outer circumference ring 16 is abutted to the outerstamper holder 15, the outer circumference ring 16 is pressed and movedtoward the movable die 7 along the backward direction a₂, as shown inFIG. 6. And, the molding surface 35 of the outer circumference ring 16is moved along the backward direction shown by the arrow a₂ such thatthe molding surface 35 protrudes from the molding surface 26 of themovable die 7 by the predetermined protrusion length t₀ which isapproximately equal to the thickness of the disc substrate 2.

When the molding die 1 is clamped, an injection molding apparatus, notshown, injects molten resin material thorough the spool bush 29 into thecavity 8 enclosed by the molding surface 35 of the outer circumferencering 16 which protrudes from the molding surface 26 of the movable die 7by the predetermined protrusion length t₀, main surface of the stamper13, and molding surface 26 of the movable die 7. After the injection ofmolten resin material into the cavity 8 is completed, the punch 20 ismoved along the forward direction shown by the arrow a₁ to cut off agate around a center hole of the disc substrate 2. After cooling downthe molten resin material, the molding die 1 is opened.

When the molding die 1 is opened, the movable die 7 is moved away fromthe stationary die 6 along the backward direction shown by the arrow a₂,and the molding surface 35 protrudes from the molding surface 26 of themovable die 7 by the maximum protrusion length t₁, as show in FIG. 7.When the molding die 1 is opened, the disc substrate 2 is moved togetherwith the movable die 7 away from the stationary die 6 along the backwarddirection shown by the arrow a₂.

Then, since the outer circumference of the disc substrate 2 is retainedby the retaining portion 36 of the outer circumference ring 16, themolding surface 35 of the outer circumference ring 16 which protrudes bythe maximum protrusion length t₁ moves the outer circumference of thedisc substrate 2 along the forward direction shown by the arrow a₁, asshown in FIG. 7. Thus, the main surface of the disc substrate 2 issatisfactorily separated from the molding surface 26 of the movable die7.

Then the molding die 1 has the pin 21 and sleeve 19 moved along theforward direction shown by the arrow a₁ to eject the disc substrate 2from the movable die 7. Thus ejected disc substrate 2 is held and takenout by a holding means, not shown, having a sucking means for suckingthe disc substrate 2, and is carried to the next processing step.

The disc substrate 2 thus molded by the molding die 1 is inspected usinga polarizing plate, etc. The result is that there is generated noseparation pattern between the outer circumference and the innercircumference of the disc substrate 2. So, the molded disc substrate 2can be prevented from generating a separation pattern when the moldeddisc substrate 2 is separated from the molding die 1. Thus, the opticaland electrical characteristics of a magneto-optical disc using the discsubstrate 2 can remain stable and desirably secured.

As has been described above, the molding die 1 includes the outercircumference ring 16 which has formed thereon the retaining surface 39for retaining the outer circumference of the disc substrate 2. So, whenthe outer circumference ring 16 is moved along the forward directionshown by the arrow a₁, the outer circumference ring 16 can release thewhole main surface of the disc substrate 2 uniformly from the moldingsurface 26 of the movable die 7 and appropriately separate the discsubstrate 2. So, the disc substrate 2 can be prevented from generating aseparation pattern on the main surface thereof.

Above-described outer circumference rings 16, 51, 61 have formed thereonretaining portions 36, 62 located halfway at the molding surface 35along the thickness direction of the molded disc substrate 2 which haveretaining surfaces 39, 65. On the other hand, the retaining surface maybe so formed as to be at approximately the same level as the moldingsurface 26 of the movable die 7, and another example of an outercircumference ring having such a retaining surface will be explainedhereinafter.

FIGS. 10 and 11 show another example of an outer circumference ring,which will be indicated with a reference numeral of 71. As shown, thereis also formed a molding surface 72 for molding an outer circumferenceend of a disc substrate 70. The molding surface 72 has a retainingsurface 73 which is formed at the same level as the molding surface 26of the movable die 7, and an inclined surface 74 so formed as to beinclined along the thickness direction of the disc substrate 70, asshown in FIG. 10. The inclined surface 74 is so formed as to face thestamper 13 and make the diameter of the molded disc substrate 70gradually increased when getting closer the stamper 13.

The retaining surface 73 retains the outer circumference of the discsubstrate 70. So, as shown FIG. 11, when the outer circumference ring 71is moved along the forward direction shown by the arrow a₁, the outercircumference ring 71 can release the whole main surface of the discsubstrate 70 uniformly from the molding surface 26 of the movable die 7,and appropriately separate the disc substrate 70. So, the disc substrate70 can be prevented from generating a separation pattern on the mainsurface thereof.

As in the above, the molding die apparatus and method for molding a discsubstrate according to the present invention is employed in molding adisc substrate of a magneto-optical disc as a disc-shaped recordingmedium by injection molding. On the other hand, the molding dieapparatus and method for molding a disc substrate according to thepresent invention can also be employed in molding a disc substrate ofother types of disc-shaped recording medium such as a CD (Compact Disc),CD-R (Recordable), CD-RW (ReWritable), and DVD (Digital Versatile Disc).

1-3. (canceled)
 4. A method for molding a disc substrate by injectionmolding by the use of a pair of stationary die and movable die formolding a disc substrate of a disc-shaped recording medium, and an outercircumference ring for molding an outer circumference end of the discsubstrate which is so mounted as to be movable along forward andbackward directions in parallel with a moving direction of the movabledie toward the stationary die, the method for molding a disc substratecomprising the steps of: moving the outer circumference ring alongbackward direction away from the stationary die to cause the outercircumference ring to protrude by a protrusion length which isapproximately equal to the thickness of the disc substrate so as toperform injection molding at the time of die clamping; moving the outercircumference ring along forward direction toward the stationary die tocause the outer circumference ring to protrude so as to separate a mainsurface of the disc substrate by a predetermined amount from a moldingsurface of the movable die for molding the main surface of the discsubstrate by the use of a retaining surface for retaining the outercircumference of the molded disc substrate which is formed at the innercircumference of the outer circumference ring when die opening isinitiated; and ejecting the disc substrate from the molding surface ofthe movable die after the die opening is finished.
 5. A disc-shapedrecording medium which has a retaining surface being of a continuous ordiscontinuous ring-shaped surface around the outer circumference thereofwhich is formed by an outer circumference ring of a molding dieapparatus for molding an outer circumference end of a disc substrate,and is retained by the outer circumference ring.
 6. The disc-shapedrecording medium as set forth in claim 5, wherein the retaining surfaceis so formed as to be in parallel with a main surface of the discsubstrate, and is located halfway along the thickness direction of thedisc substrate or at the same level as the main surface of the discsubstrate.